1. Field of the Invention
This invention relates broadly to cellular wireless communication networks. More particularly, this invention relates to a methodology and systems for identification and measurement of interference in Global System for Mobil Communications (GSM) cellular wireless networks.
2. State of the Art
Because cellular wireless communication networks re-use frequency across geographic areas, all cellular wireless communication networks contain interference (both co-channel and adjacent channel). All modern-day wireless protocols, including the GSM protocol, take this into consideration. However, it is important for cellular network carriers to manage interference to its minimum possible levels because interference within a network reduces capacity (the number of subscribers, or amount of traffic, a network can accommodate). Thus, in order to maximize the amount of revenue a network can generate, maximize quality of service, and to minimize the capital expenditures necessary to support that revenue (i.e. purchasing new base stations), it is critical that the network interference be minimized.
The current solutions for optimizing cellular wireless networks involve a process of gathering network data and processing that data to determine the best possible optimization of network variables to minimize interference. The data can come from a number of sources, but drive testing is the most accurate. Drive testing is the process of driving the roads in a given market with a piece of test equipment that typically includes a laptop computer integrated with a wireless terminal, a GPS receiver and a demodulating scanning receiver. Once the drive test data is collected, the data is typically provided to post-processing tools which apply various mathematical algorithms to the data to accomplish network planning and optimization. An example of post-processing is automatic frequency planning (AFP), where the data is processed to determine the optimal arrangement of frequencies to cell site sectors to minimize network interference. Another post-processing application is automatic cell planning (ACP) which analyzes network variables to aid network engineers in making decisions on how best to minimize interference in the network. For GSM networks, these network variables include: the frequencies per cell site sector, the cell site antenna's height and/or azimuth and/or tilt, the cell site sector's transmission power, cell site locations or new cell site locations, and a host of other variables that impact radio frequency propagation.
When analyzing a cellular wireless system, it is important that such analysis be able to distinguish between signals originating from different base stations. Two phenomena make such separation difficult: co-channel interference and adjacent-channel interference. Co-channel interference occurs when transmitters in a given area use the same frequency channel. Adjacent-channel interference occurs when base stations in a given area transmit on adjacent channels.
A number of techniques have been developed to achieve the stated goal of signal separation. One class of techniques associate signals with transmitting base stations based on the ability to decode base station identifiers (also referred to as color codes) in the transmitted signals. If the base station identifier can be detected, the signal is ascribed to the nearest base station with this base station identifier. These techniques require measuring position of the measurement instrument as well as a priori knowledge of the network geographical layout and the assignments of identifiers to the base stations of the network. Moreover, these techniques are ineffective in the presence of interference (either co-channel interference or adjacent-channel interference) because base station identifiers cannot be detected.
Another technique involves joint-decoding of the color code signal components with channel estimation for each signal path. This technique, which is described in detail in U.S. Pat. No. 6,324,382, relies on accurate estimation of the transmission channel characteristics for the signal paths from each interfering base station. In practice, this technique suffers from poor decoding performance in addition to its low measurement speed.
An improved technique is described in U.S. Patent Application Publication US2001/0034208, published Oct. 25, 2001, commonly assigned to the assignee of the present invention, incorporated by reference herein in its entirety. This technique uses correlation with known signal patterns (for example, synchronization and training sequences), which yields a significant processing gain. This gain allows signal detection in the presence of interference even when its level is substantially below the level of one or more interfering signals. Signal identification (i.e., association with transmitting base stations) is based upon the Global Positioning System (GPS) position of the measurement instrument and time-of-arrival of individual Frequency Correction Channel (FCCH) correlation peaks at different measurement points. The power level of the signal at a given FCCH peak is stored in a database together with its time of arrival. When color code decoding is successful, all instances of the given FCCH peak during its lifetime in the database are back-annotated with the newly-found color code. This technique provides improved signal detection in the presence of interference; however, it requires successful color code decoding associated with a given FCCH peak for the instances of the given FCCH peak to be back-annotated with the decoded color code. Moreover, it is possible for the same color code to be used by different base stations. In this case, the identification of base stations based on color codes may not provide unique base station identification, and thus require complex post processing to resolve such situations. Finally, it relies on the difference between time-of-arrivals for FCCH peaks to identify base stations corresponding thereto in the event that the FCCH peaks never have a color code decoded from them during a given session. Thus, data measured and stored for multiple sessions or with multiple instruments cannot be efficiently associated between them. The present invention builds upon the methodology and apparatus described in U.S. Patent Application Publication US2001/0034208 to provide a more efficient solution and add additional features not described therein.